A subterranean well hydraulically actuated double acting piston and cylinder actuator assembly may be used for performing various functions in an oil or gas well. Proper operation of the actuator assembly requires that specific quantities of hydraulic fluid be pumped into the assembly for moving the position of the piston relative to the cylinder to a precise position. The hydraulic actuator assembly is remotely controlled by first and second hydraulic lines connected to opposite sides of the assembly. One such actuator is a SCRAMS hydraulically controlled actuator manufactured by, or available from, PES Incorporated of the Woodlands, Texas, and is useful as described in U.S. Pat. No. 5,957,207 for separately controlling two or more producing zones in an oil or gas well. The location of the piston relative to the cylinder must be stroked to a precise position. The SCRAMS tool an electronic position sensor device is used to determine the position of the actuator. Electronic devices often fail in such service due to the severe ambient conditions. It is desirable, therefore, to have an alternative means to determine or control the stroke of the actuator assembly via the hydraulic fluid control lines, outside of the well.
The volume of hydraulic fluid used to actuate or vented from the actuator assembly can in principle be used to determine the position of the piston relative to the cylinder. In use, the actuator assembly is controlled from a remote surface facility through the first and second elastic hydraulic lines, such as thermoplastic lines, which, because of their great length and material, are elastic and therefore the elastic volumetric changes in the actuating line during operation make volumes measured at the surface unreliable.